In the research or application of liquid crystals, the control of the rotational direction of liquid crystal molecules is an extremely important step and is generally referred to as liquid crystal alignment. The most common liquid crystal alignment method is coating alignment materials on films formed on the surface of the substrates, and then making the molecules of the alignment films to generate directional arrangements by means of rubbing. When liquid crystal molecules attract the alignment films and form an ordered liquid crystal arrangement in virtue of intermolecular forces, the effect of liquid crystal alignment is finally achieved. The method of rubbing alignment can provide strong alignment ability for liquid crystal molecules. However, during the process of rubbing, contaminations of static electricity, scratch, particles and dust may be resulted due to the friction of rubbing cloth. These contaminations often directly cause damages in liquid crystal devices resulting in a decrease in the product yields.
Therefore, in academia and industry, non-contact alignment methods are being developed for easily controlling the alignment mode of liquid crystal molecules and for avoiding the contaminations of static electricity and particles. The most widely applied non-contact alignment method is photo alignment, such as ultraviolet light alignment.
The photo alignment utilizes linear polarized ultraviolet light to irradiate on macromolecular polymer alignment films containing photosensitizers, allowing the macromolecular polymer to have the ability of alignment. The photo alignment is able to avoid contaminations on the surface of glass substrates, to align in a small area, to make patterned alignment through photomasks, and to control the parameters of liquid crystal units such as pretilt angle and anchoring energy, etc. by the angle of incident light and irradiation time. The photo alignment material is a polymer formed by bonding many monomer molecules.
Generally alignment materials for the photo alignment may be divided into three groups: the first group is photo crosslinking reaction: After the alignment film is irradiated by the linear polarization ultraviolet light, molecules in the direction of polarization will be bonded to form long-bond molecules, allowing the alignment film to have an anisotropic distribution, and the liquid crystal molecules are consequently arranged along with the direction of the long-bond molecules. The second group is photo degradation reaction: after the alignment film is irradiated, the long-bond molecules in the direction of polarization will be degraded by the ultraviolet light, making the alignment film to form an anisotropic distribution, therefore the liquid crystal molecules will be arranged along the direction of the undegraded long-bond molecules. The third group is photo isomerization (cis-trans): generally an alignment material containing azo molecules is used, and the alignment material will form different arrangement directions before and after irradiation.
However, the process of photo alignment method is complicated and has a high cost for the necessary raw materials. Thus, this method is not suitable for large scale production.